This invention relates to a shift conversion unit for causing reformed gas, which has been produced by reforming hydrocarbon-based feed gas by partial oxidation reaction, to undergo shift conversion by water gas shift reaction with catalyst.
In general, hydrogen can be produced by reforming hydrocarbon or methanol. Fuel reforming units for producing hydrogen through such reforming can be used for fuel cells, hydrogen engines or the like.
As a reforming unit of such kind, there is conventionally known one which is incorporated into a fuel cell system as disclosed in Japanese Unexamined Patent Publication No. 11-67256. This fuel reforming unit includes a fuel reformer loaded with catalyst which exhibits activity to partial oxidation reaction, and is designed to introduce feed gas into the fuel reformer to produce reformed gas with hydrogen by partial oxidation reaction of the feed gas.
Further, in order to reduce CO (carbon monoxide) in the reformed gas produced in the above manner and improve the yield of hydrogen, the reformed gas is generally caused to undergo shift conversion by subjecting it to water gas shift reaction with shift conversion catalyst in a shift reaction section of a shift conversion unit.
Specifically, in the water gas shift reaction, carbon monoxide is oxidized by water to convert into carbon dioxide and hydrogen as expressed in the following chemical formula.
CO+H2Oxe2x86x92CO2+H2
Meanwhile, for the shift conversion unit of this kind, its shift reaction section has poor heat resistance and therefore cannot receive the reformed gas as supplied at high temperature (for example, 700xc2x0 C.) from its reforming reaction section and cause such high-temperature reformed gas to undergo reaction. Accordingly, the unit is designed to divide the shift reaction section into high-temperature and low-temperature shift reaction sections, first introduce the reformed gas from the reforming reaction section into the high-temperature shift reaction section after decreasing its temperature down to for example 400xc2x0 C. and then introduce the reformed gas having exited from the high-temperature shift reaction section into the low-temperature shift reaction section after further decreasing its temperature down to for example 200xc2x0 C.
In this case, however, there is the need for controlling respective inlet temperatures of the reformed gas flowing into the high-temperature and low-temperature shift reaction sections. This creates the problem of complicating the component layout for satisfying the need.
Further, under the temperature conditions where the reaction speed is high as in the above case, the reaction cannot be realized. Therefore, the above unit cannot avoid that the temperature range within which the reformed gas can undergo shift conversion is limited narrowly.
Furthermore, in the water gas shift reaction under the high temperature conditions, it is necessary to increase the amount of catalyst in order to ensure the heat resistance of the catalyst. This correspondingly increases the thermal capacity of the shift reaction section thereby causing the problem of deteriorating its response to load variations and start-up characteristics.
The present invention has been made in view of these problems and therefore an object thereof is to enable the high-temperature reformed gas from the reforming reaction section to undergo shift reaction in the shift reaction section just as it stands by contriving the construction of the shift conversion unit and to thereby simplify the construction of the shift conversion unit.
To attain the above object, in the present invention, the shift reaction section of the shift conversion unit subjects the reformed gas from the reforming reaction section to shift reaction while heat-exchanging it with feed gas or heat recovery gas toward the reforming reaction section.
More specifically, the present invention is directed to a shift conversion unit having a shift reaction section (10) for causing hydrogen-rich reformed gas produced by reaction including partial oxidation of feed gas in a reforming reaction section (6) to undergo shift conversion by water gas shift reaction with shift conversion catalyst. In this shift conversion unit, the shift reaction section (10) is arranged to introduce the reformed gas from the reforming reaction section (6) directly into a reformed gas passage (11) and effect the shift reaction while heat-exchanging the reformed gas with the feed gas.
Thus, the high-temperature reformed gas from the reforming reaction section (6) is introduced directly into the shift reaction section (10) and in the shift reaction section (10), the reformed gas is then caused to undergo shift conversion by water gas shift reaction while undergoing heat exchange with the feed gas in the feed gas passage (3) which should be supplied to the reforming reaction section (6). Accordingly, the reformed gas having exited from the reforming reaction section (6) will undergo shift conversion while keeping its high temperature. Therefore, the reformed gas can undergo shift conversion over a wide temperature range from high temperature conditions where the reaction speed is high to low temperature conditions where the reaction speed is low but the gas reacts advantageously at equilibrium.
Further, the need to control the temperature of the reformed gas can be eliminated, thereby simplifying the construction of the shift conversion unit.
Furthermore, the loading amount of the shift conversion catalyst into the shift reaction section (10) can be decreased and the thermal capacity can be reduced correspondingly. As a result, the shift reaction section (10) can maintain excellent response to load variations and start-up characteristics.
The shift conversion catalyst of the shift reaction section (10) is preferably noble metal catalyst with heat resistance or catalyst in which Pt, Pt alloy or Ru alloy is used as active metal. This provides desirable shift conversion catalyst for effecting shift reaction at the high temperature. In other words, if the noble metal catalyst with heat resistance is used, the catalyst can exhibit excellent endurance and hold high activity over a wide temperature range. Alternatively, if the catalyst in which Pt, Pt alloy or Ru alloy is used as active metal is employed, the catalyst can exhibit high activity at high temperatures and makes it difficult to cause methanation.
The shift conversion catalyst of the shift reaction section (10) can be applied to or supported on porous material. Since the porous material has a large surface area, the use of this material can increase the contact area between the shift conversion catalyst and the reformed gas in the shift reaction section (10) to increase the reaction rate and improve the efficiency of heat radiation.
The porous material is preferably of either foam metal, cordierite or ceramics. In this case, there can be obtained desirable porous material especially for ensuring the increase in the contact area of the catalyst with the reformed gas.
In the vicinity of the shift reaction section (10), a feed gas passage (3) can be provided for supplying the feed gas to the reforming reaction section (6). With this arrangement, the feed gas in the feed gas passage (3) located in the vicinity of the shift reaction section (6) is heated by heat of reaction in the shift reaction section (10). Accordingly, the heat of reaction in the shift reaction section (10) can be recovered for the preheating of the feed gas and this self-recovery of heat can improve the thermal efficiency of the shift conversion unit.
In the above case, the shift reaction section (10) and the feed gas passage (3) can be integrally formed in a housing (1). With this arrangement, the construction of the shift conversion unit can be simplified, thereby providing cost reduction.
A heat exchanger (15) may be provided for exchanging heat of reaction and sensible heat in the shift reaction section (10) with heat of the feed gas in the feed gas passage (3) by heat radiation. In this manner, the rate of heat exchange can be increased between the shift reaction section (10) and the feed gas thereby improving the efficiency of heat transfer.
The reformed gas passage (11) of the shift reaction section (10) is preferably formed so that the reformed gas flows from the center side toward the outer peripheral side of the shift reaction section (10). With this arrangement, such a temperature profile in the shift reaction section (10) can be formed that the temperature differs from entrance to exit thereof.
In the above case, the distance of portion of the shift reaction section (10) located downstream in a direction of flow of the reformed gas to the feed gas passage (3) is preferably larger than that of portion of the shift reaction section (10) located upstream in the direction of flow of the reformed gas to the feed gas passage (3). With this arrangement, the rate of heat exchange of the shift reaction section (10) with the feed gas passage (3) by heat radiation varies between the sides of the shift reaction section (10) upstream and downstream in the flow direction of the reformed gas. Accordingly, the temperature at the exit of the shift reaction section (10) can be held substantially uniformly.
The heat exchanger (15) can include a heat transfer fin (16) presented to the feed gas passage (3). In this case, the efficient of heat transfer can be further improved.
It is preferable that a plurality of said heat transfer fins (16) are provided along the feed gas passage (3) and the pitch of some of the heat transfer fins (16) located upstream in the direction of flow of the reformed gas in the shift reaction section (10) is smaller than that of some of the heat transfer fins (16) located downstream in the direction of flow of the reformed gas. With this arrangement, heat exchange between the shift reaction section (10) and the feed gas can be made smoothly.
A heat exchanger (23) may be provided which includes a reformed gas side heat transfer fin (21) presented to the reformed gas passage (11) and a feed gas side heat transfer fin (22) presented to the feed gas passage (3) and exchanges heat of reaction and sensible heat in the shift reaction section (10) with heat of the feed gas in the feed gas passage (3). Further, the shift conversion catalyst of the shift reaction section (10) is applied to or supported on at least the reformed gas side heat transfer fin (21). With this arrangement, the reformed gas in the reformed gas passage (11) of the shift reaction section (10) undergoes the shift reaction through the contact with the shift conversion catalyst on the reformed gas side heat transfer fin (21) presented to the reformed gas passage (11). The resultant heat of reaction is transferred from the reformed gas side heat transfer fin (21) to the feed gas in the feed gas passage (3) through the feed gas side heat transfer fin (22). Also in this case, the efficiency of heat transfer from the shift reaction section (10) to the feed gas can be improved.
The reforming reaction section (6), the feed gas passage (3) and the shift reaction section (10) may be integrally provided in a housing (1). In this manner, the construction of the shift conversion unit can be further simplified, resulting in cost reduction.
Alternatively, in a shift conversion unit of the present invention which has a shift reaction section (10) for causing hydrogen-rich reformed gas produced by reaction including partial oxidation of feed gas in a reforming reaction section (6) to undergo shift conversion by water gas shift reaction with shift conversion catalyst, the shift reaction section (10) is arranged to effect the shift reaction while heat-exchanging the reformed gas from the reforming reaction section (6) with heat recovery gas.
With this arrangement, the high-temperature reformed gas from the reforming reaction section (6) is caused in the shift reaction section (10) to undergo shift conversion by water gas shift reaction while undergoing heat exchange with the heat recovery gas. Accordingly, the reformed gas having exited from the reforming reaction section (6) will undergo shift conversion while keeping its high temperature. Therefore, the reformed gas can undergo shift conversion over a wide temperature range from high temperature conditions where the reaction rate is high to low temperature conditions where the reaction rate is low but the gas reacts advantageously at equilibrium.
Specifically, through the heat exchange between the high-temperature reformed gas from the reforming reaction section (6) and the heat recovery gas, the reformed gas entrance side of the shift reaction section (10) is elevated in temperature to increase the reaction rate while the reformed gas exit side thereof is lowered in temperature to reduce the reaction rate. As a result, the CO concentration can be reduced at thermal equilibrium.
Further, the need to control the temperature of the reformed gas can be eliminated thereby simplifying the construction of the shift conversion unit.
Furthermore, since the heat exchange is made in the shift reaction section (10), high-temperature heat exhausted therefrom can be recovered as heat recovery gas.
In addition, the loading amount of the shift conversion catalyst into the shift reaction section (10) can be decreased and the thermal capacity can be reduced correspondingly. As a result, the shift reaction section (10) can maintain excellent response to load variations and start-up characteristics.
In this case, like the aforementioned case, the shift conversion catalyst of the shift reaction section (10) may be noble metal catalyst with heat resistance. Since the noble metal catalyst with heat resistance exhibits excellent endurance, it can hold high activity over a wide temperature range.
Further, the shift conversion catalyst of the shift reaction section (10) may be catalyst in which Pt, Pt alloy or Ru alloy is used as active metal. If this catalyst in which Pt, Pt alloy or Ru alloy is used as active metal is employed, the catalyst can exhibit high activity at high temperatures and makes it difficult to cause methanation.
The shift conversion catalyst of the shift reaction section (10) is applied to or supported on porous material. With this structure, the contact area between the shift conversion catalyst and the reformed gas in the shift reaction section (10) can be increased to increase the reaction rate and improve the efficiency of heat radiation.
The porous material is preferably of either foam metal, cordierite or ceramics. In this case, there can be easily obtained porous material that especially ensures to increase the contact area with the reformed gas.
The shift conversion catalyst of the shift reaction section (10) may be applied to or supported on a catalyst support of metal. Thus, there can be obtained a desirable catalyst support for cooling the catalyst presented to the reformed gas passage (11) through the heat exchange with the heat recovery gas.
A heat recovery gas passage (37) through which the heat recovery gas flows can be provided in the vicinity of the catalyst support. With this arrangement, since the catalyst support is surrounded by the heat recovery gas passage (37), the thermal efficiency can be improved.
The heat recovery gas can be air. If air is the recovery gas, stable heat exchange can be implemented even at partial loads in the case of recovery of high-temperature heat, thereby easily obtaining serviceable heat recovery gas.
Further, the heat recovery gas may be off-gas from an oxygen electrode (34) (air electrode) of a fuel cell (31). If the off-gas of the fuel cell (31) is used as the heat recovery gas in this manner, it is not necessary to newly prepare air as the heat recovery gas unlike the above case, and the existing off-gas of the fuel cell (31) can be utilized as it is. In addition, the need for any blower and its driving power for allowing the air to flow as the heat recovery gas can be eliminated.